1. Field of the Invention
This invention relates to the chemical analysis of substances, and more particularly, to a method and apparatus for the potentiometric or radiometric analysis of a test element having a biological fluid deposited thereon.
2. State of the Prior Art
Chemical analyzers have been developed for performing quantitative chemical analyses of biological fluids, such as blood serum, to enable the physician to obtain a more complete picture of a person's physical condition. In one type of such apparatus, reagents are added to a sample of blood serum, and after sufficient incubation, a color change or a fluorescence is sensed by a radiometer. In a second type of apparatus, certain ionic blood components are detected potentiometrically by measuring the potentials generated by ion-selective electrodes (hereinafter referred to as ISE's). Included in such tests are the ions Cl.sup..crclbar., Na.sup..sym., K.sup..sym. and HCO.sub.3.sup..crclbar..
Apparatus for measuring various ions in solution typically include a reference electrode and an ISE which incorporates a reference half-cell, generally comprising a solution of known ion activity. When the reference electrode and the ISE are immersed in a solution to be analyzed, an electrochemical cell is formed and a potential developes across the electrodes of the cell. This potential is proportional to the logarithm of the activity of ions in solution to which the ISE is sensitive. The logarithmic relation is described by the well-known Nernst equation.
Most of the commercially-available apparatus for testing the electrolyte concentrations in a biological fluid utilize liquid reagents and require analyzer equipment having intricate solution handling and transport capabilities. Such apparatus is complex and expensive, requires skilled operators, and must be used in a laboratory setting where suitable power and plumbing facilities are available. Further, known prior-art devices normally require large amounts of the sample fluid to be tested, since they utilize relatively large, bulky electrodes which must be immersed in the sample fluid. Electrodes of the type described are disclosed in U.S. Pat. No. 3,502,560 to Wise, issued Mar. 24, 1970, and U.S. Pat. No. 3,575,834, to Hoole et al., issued Apr. 20, 1971.
The measuring of electrolyte concentrations in blood serum is an important tool in certain clinical analyses which must be performed under emergency conditions. For example, the concentration of sodium ions in the blood can be an indication of dehydration, and an elevated potassium concentration can indicate a weak cardiac muscle which can stop at any time. Thus, a need has existed for a portable, compact analyzer which is adapted to perform emergency tests in a variety of locations, such as in an ambulance, in an operating room, or in the physician's office or clinic. None of the prior-art devices, which require complex solution handling devices and use large bulky electrodes, are suitable for such use.
Recent developments have provided ISE's in essentially planar, dried form, suitable for use in pairs in an analyzer. An example of such a device is described and claimed in commonly-owned U.S. Pat. No. 4,053,381, issued Oct. 11, 1977, entitled "Device for Determining Ionic Activity of Components of Liquid Drops."